The prior art of probing ultrasonic fields in liquids involves the use of miniature hydrophones consisting typically of a small crystalline or ceramic piezoelectrically active element, which is mounted together with suitable backing at the end of a tube or needle, or other similar supporting structure. Despite their small size, such hydrophones unavoidably alter the acoustic field at the probed point because of the large difference in acoustic impedance between the hydrophone materials and the liquid medium in which the hydrophone is immersed during use. Furthermore, material and geometric factors of the sensing element and supporting structure lead to multimode response, undesirable reflections and a complicated frequency and angle dependence of the response.